1. Field of the Invention
This invention relates to stops for coherent beams of radiation.
2. Description Relative to the Prior Art
If a coherent beam of radiation, such as is generated by a gas or solid state laser, is truncated by a stop, for example, stop 18 illustrated in FIG. 1a of the accompanying drawings, which consists of a hole 20 in a piece 22 of metal, the sharp edge 24 of the hole 20 causes the beam to diffract at the edge of the hole. FIG. 1b is a plot of the optical density of the stop 18 against the position x across the stop, and FIG. 1c is a plot of the % light passed against position x across the stop illustrated in FIG. 1a . The diffraction at the edge of the hole causes an interference pattern in the beam downstream of the hole 20. The presence of the interference pattern in the beam is undesirable for many purposes. For example, if the laser and the stop were to be used in a laser printer, the interference pattern would cause the size of the stop on the photoreceptor to be increased. At least in high resolution printers, a small spot size is desirable and such an increase in the spot size would be undesirable. To avoid any interference-producing truncation of the beam, all of the optical elements would have to be made very large, with consequent high cost. The undesirable interference patterns caused by beam truncation are encountered, and are undesirable, also in laser fusion systems, video disk players and recorders, inter alia. Thus, it is apparent that there is a need to minimize the interference effects caused by truncating a laser beam.
U.S. Pat. No. 3,935,545 describes a method and apparatus for reducing diffraction-induced damage in high power laser amplifier systems. In particular, it discloses appropriately tailoring a laser beam profile by passing the beam through an aperture having a uniformly high optical transmission within a particular radius and a transmission which drops gradually to lower and lower values at progressively greater radii. An example of such a stop is stop 26 illustrated in FIG. 2a of the accompanying drawings which consists of a piece 28 of clear sheet glass which has been rendered opaque in the cross-hatched region 30. In the central, disc-shaped area 32 there is 100% transmission, i.e. the stop is clear in the area 32. In the annulus 34, which is hatched in FIG. 2a, the transmission varies from 100% adjacent the clear area 32 to 0% adjacent the opaque region 30. Thus, there is a gradient in the optical density along radii of the aperture, beginning at the outer radius of the disc-shaped area 32 and ending at the inner radius of the opaque region 30. Such an aperture may be described as having a soft edge, and its transmission efficiency, i.e. % light "passed", is illustrated in FIG. 2b.
Another method of fabricating soft-edge apertures, which is commoly in use, involves specialized thin film patterning techniques in which a thin film of chrome or other opaque material is deposited on a glass substrate. The soft aperture is patterned into the film during deposition by a mechanical device to provide a gradient in the optical density at the edge of the aperture.
The fabrication of known soft-edge aperture is difficult and expensive.